The planet Neptune was discovered in 1846 after mathematicians, using Newton's theory of gravitation, predicted exactly where it would be found. This discovery was hailed as a triumph for modern physics. Soon after the discovery of Neptune, however, astronomers found apparent irregularities in its orbit and began searching for another planet, deemed 'Planet X', beyond Neptune. After decades of searching, Pluto was finally discovered 1930 by astronomer Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona.
Now after 76 years, it seems to many as if the planet Pluto has been UN-discovered: what's going on here? Astronomers now classify Pluto as a 'dwarf planet' along with Eris, formerly known as Xena, and Ceres, formerly known as the largest asteroid. Why did they do this?
To understand Pluto's apparent demotion, it is necessary to understand the reason scientists classify any objects that they are studying. For instance, geologists classify rocks as igneous, sedimentary, or metamorphic based on their structure and composition. Grouping rocks in this manner is a key toward understanding the origin of rocks and the relationships between rocks of the different categories in the 'rock cycle'. Classifying objects helps scientists understand the connections that exist within a group and also helps them discern relationships with objects in other categories. This same principle holds for celestial objects.
By 1940 is was clear that Pluto was a very different kind of planet. It was too small and had an elliptical orbit unlike any other planet. Since it was not rocky like Earth nor gaseous like Jupiter, Pluto seemed more likely to be an escaped moon of Neptune than a planet. In the 1960's some astronomers wanted to re-classify Pluto as a member of a new class of solar system bodies. By the 1990's astronomers knew that Pluto was much smaller than the Earth's moon and composed mostly of ice like comets or the newly discovered Kuiper belt objects which exist beyond Neptune.
In July of 2005 the issue finally came to a head when a 'tenth planet', temporarily named 'Xena' was discovered beyond Pluto. A day after this discovery another large body was reported as the 11th planet and within 12 months astronomers were forced to consider nearly a dozen other large ice bodies as planets. In August of 2006, the International Astronomical Union (IAU) met to clarify the issue of new planets and decided to set up a new category of solar system bodies, called dwarf planets, to accommodate Pluto and Xena. This new grouping recognizes the differences between dwarf planets and the eight other large planets that orbit the Sun. Astronomers expect to discover dozens more dwarf planets over the next decade.
This new grouping helps astronomers better understand the origin of all Pluto-like objects when the solar system formed 4.55 billion years ago. Classification and re-classification (as new data becomes available) help science progress to better understanding.
On a side note, it is interesting that the 'Pluto furor' has attracted more interest in astronomy than the ongoing search for life on Mars or the new Hubble discoveries for the Big Bang. Somehow that must have something to do with the name Pluto being associated with a cartoon character. If astronomers demoted Uranus, would anyone care?
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